Lineage specific differentiation of pluripotent cells in vitro: a role forextraembryonic cell types

The controlled differentiation of pluripotent cells will be a prerequisite for many cell therapies. We have previously reported homogeneous conversion of embryonic stem (ES) cells in vitro to early primitive ectoderm-like (EP L) cells, equivalent to early primitive ectoderm, an obligatory differentia tion intermediate between ES cells and somatic cell populations. Early prim itive ectoderm-like cells differentiated within aggregates form mesodermal lineages at the expense of ectoderm. In this work we demonstrate that the f ailure of EPL cells to form ectodermal cell types does not reflect an inher ent restriction in developmental potential. Early primitive ectoderm-like c ells form ectodermal derivatives such as neurons in response to neural indu cers such as retinoic acid, or when differentiated in the environment provi ded by ES cell embryoid bodies. This could be explained by signals from the extraembryonic cell type visceral endoderm which forms in differentiating ES cell but not EPL cell aggregates. Consistent with this possibility, cult ure of EPL cell aggregates in the presence of visceral endoderm-like signal s did not prevent differentiation of the pluripotent cells, but resulted in suppression of mesoderm formation. These results suggest a role for viscer al endoderm in regulation of germ layer specification from pluripotent cell s, and can be integrated into a model for cell differentiation in vitro and in vivo.